Neoplastic development of Syrian hamster embryo (SHE) cells is a multistep process. We have examined the influence of chemical carcinogens and the role of cellular and viral oncogenes in this process. We have compared the susceptibilities of normal and carcinogen-induced preneoplastic SHE cells to neoplastic transformation following transfection by the calcium phosphate precipitate techniques with plasmids of genomic clones of four oncogenic viruses: polyoma virus, Harvey murine sarcoma virus (Ha-MSV), Rous sarcoma virus (RSV) and MC29 virus (pSVv-myc). Normal SHE cells transfected with polyoma virus DNA formed progressively growing tumors of hmaster origin within 3-4 weeks when injected into nude mice. In contrast, SHE cells treated with Ha-MSV DNA remained nontumorigenic. SHE cells treated with RSV DnA formed one tumor (one of six sites) with a latency period of 15 weeks. SHE cells transfected with either Ha-MSV DNA and pSVv-myc DNA or RSV DNA and pSVv-myc DNA formed tumors with short latency periods. Polyoma virus DNA, Ha-MSV DNA or RSV DNA could individually neoplastically transform preneoplastic SHE cells which were immortalized following treatment with the carcinogen diethylstilbestrol. These results suggest that multiple changes or activated oncogenes are required for the neoplastic transformation of SHE cells. To determine if normal cellular factors or genes can regulate the phenotypic expression of tumorigenicity and/or oncogenes, cell-cell hybrids between chemically transformed SHE cells and either normal or preneoplastic SHE cells were prepared. Our results indicate that anchorage independence, which is a good marker for tumorigenicity of these cells, is suppressed in hybrids between tumorigenic and normal cells and in hybrids between tumorigenics and most but not all preneoplastic cells. This suggests that this suppressive ability may be lost during neoplastic progression and represents one step in this process.